1. Field of the Invention
The present invention relates to thermoelectric materials, and, more particularly to thermoelectric materials with low thermal conductivity, high electrical conductivity and a high figure of merit.
2. Description of Related Art
Today's thermoelectric (TE) device requires new compound materials with a high Seebeck coefficient, a high electrical conductivity (EC) and a low thermal conductivity (TC). Among the various TE materials that have been demonstrated thus far, the highest figure of merit for TE materials (“ZT factor”) achieved is 2.5 using p-type 10 Å/50 Å Bi2Te3/Sb2Te3 superlattices. Conversely, the ZT for n-type 10 Å/50 Å Bi2Te3/Sb2Te3 superlattices is 1.46 at 300 K which is less than impressive. The performance of p-n junction devices for generators or coolers are dictated by the average value of ZT factors for both the p-type and n-type TE materials.
Good thermoelectric materials are characterized with high Z factor and its dimensionless product with the operating temperature, ZT (often called as the figure of merit for TE materials); Z=S2σ/κ and ZT=S2σT/κ, where S is the Seebeck coefficient (thermally generated open circuit voltage of material, μV/K), σ the electric conductivity (1/Ohm-cm), κ the thermal conductivity (mWatt/cm-K), and T the absolute temperature of operation (K).
Noticeable efforts to achieve high ZT have been made in searching for new TE materials that have an intrinsic high Seebeck coefficient, a high electrical conductivity, and a low thermal conductivity. Many TE materials have been brought into laboratory tests but the overall findings are less than impressive. Therefore, major efforts have been directed in part or in whole into structural modification of TE compound materials to enhance electrical conductivity while maintaining or reducing thermal conductivity. One of the examples is the superlattice structure of TE compound materials.
There are numerous applications for this potential breakthrough technology, such as power generation and active cooling devices. The cost savings by efficient TE materials with new nanovoid technology are immeasurable, especially for power generation applications for those spacecrafts in space exploration missions. The high figure of merit (ZT>5) of advanced TE generator will offer a high efficiency that may be competitive to high efficiency of most solar cells. The TE generator has much broader temperature range based on a specific TE material than band structure of solar cells. Multilayer of TE generators that cover a temperature range to another, respectively, will increase the overall efficiency, even better than the best known solar-cells. FIG. 4 shows the estimated figure of merit for the invented TE material technology.
Previously, poor TE properties of TE devices, including TE generators or TE coolers, have limited system design and application. The figure of merit (ZT) demonstrated so far is still much less than 4.0, the target value for p-n junction materials. It is well known that void structure in TE materials could improve overall TE performance. Nevertheless, most test samples with a certain void fraction have shown unsatisfactory performance due to failure in design and failure to synthesize proper nanovoid structure. For maximization of TE performance, the nanovoids need to maintain an optimized dimension comparable to the phonon mean free path so that they can reduce thermal conductivity by disrupting phonons without sacrificing electron transport.
The incorporation of nanovoids needs to enable reduction of thermal conductivity as well as increase of electrical conductivity, in order to maximize the thermoelectric figure of merit. In this regard, material design and synthesis are critical to achieving this goal since nature does not allow these two properties at the same time. Electrical and thermal conductivities usually change in the same direction, because both properties are, in most materials, originated from contribution of energetic electrons. TE materials with void structure have been studied in only a few systems, such as bismuth, silicon, Si—Ge solid solutions, Al-doped SiC, strontium oxide and strontium carbonate. One good example that showed positive influence of void incorporation was Si—Ge alloy samples prepared by conventional sintering-based method. In this case, a 30% increase in TE performance was observed with 15-20% void introduced. A recent approach to create nanoscale void structure was solution-based metalorganic deposition that involves metal precursors. Organic groups grafted to metal precursors are unstable and removed easily during heating process. The thermally-labile alkyl groups created nanovoid structure in bismuth metal film.
In the previous attempts to develop TE materials having a void structure, most of the void structures are poorly defined in terms of void size and interconnectivity. Conventional fabrication techniques don't allow a sophisticated control of nanoscale structure. Most void structures form interconnected void channels which disturb electron mobility and cause electrical failure. Typical void sizes in most of prior-art studies were in the micrometer range and thus phonon disruption was rarely observed.
An object of the present invention is to provide a thermoelectric material having a high figure of merit.
An object of the present invention is to provide a thermoelectric material having low thermal conductivity and high electric conductivity.
An object of the present invention is to provide a thermoelectric material having a void structure.
Finally, it is an object of the present invention to accomplish the foregoing objectives in a simple and cost effective manner.